1. Field of the Invention
This invention relates to coal-water slurries containing a rheological additive which is an amide obtained by the reaction of a polycarboxylic acid with a polyether diamine. A portion of the carboxyl groups may also be reacted with other amines or hydroxylic compounds. The amides are employed in an amount from about 0.1 to about 4% by weight of the coal-water slurry, the slurry having from about 60 to about 80% by weight solids--the balance being water.
2. Description of the Prior Art
In recent years there has been a great deal of interest in utilizing coal-water slurries in lieu of oil for electric power generation not only because of the lower cost of coal but also because of its availability.
Coal-water slurries have been produced which are fluid and handle in about the same way as petroleum fuels. Since the heat generated during combustion is sufficiently high, these coal-water slurries may be burned directly to generate power without dewatering. As the solids of the coal-water slurry is increased, the fuel value of the slurry also increases and it is generally not considered economically feasible to use slurries having less than about 50 to 55% solids.
One of the difficulties encountered with coal-water slurries at a solids content of about 60% and higher is that the dispersion becomes an immobile mass. Thus, for burning it has to be handled in the same manner as lump coal. Handling in this respect includes not only transportation of the coal from the mine source but also the delivery of the coal to a combustion chamber such as the firebox of a steam boiler. Unless coal-water slurries have the same liquidity as oil at these higher solids content so that they may be transported by pipeline and injected into a firebox by spraying, the advantage of using a liquid carrier (i.e., water) for the coal is lost. Stated otherwise, the particles of coal in the slurry at these higher solids levels tend to convert the liquid carrier (water) into a plastic mass whereby the advantage of employing a liquid carrier is lost.
The prior art has overcome some of these difficulties by providing additives which may be used in relatively small amounts to assure that the coal-water slurry at high solids content is fluid. For example, the use of alkali metal soaps of fatty acids is disclosed in U.S. Pat. No. 4,435,306. Numerous alkoxylated additives, including nitrogen-containing products, are also disclosed to be effective in the formulation of useful coal-water slurries. U.S. Pat. Nos. 4,358,293, 4,441,889, 4,477,259 and 4,478,603, describe the use of block polymers of ethylene and propylene oxide derived from nitrogen-containing compounds, such as ethylene diamine, and having molecular weights of at least about 14,000 as useful dispersants for the preparation of high solids content coal aqueous mixtures. Other nitrogen-containing materials, such as cocamidopropyl betaine and ammonium salts, are also disclosed in U.S. Pat. No. 4,477,259. In U.S. Pat. No. 4,398,919 coal-water slurries are prepared utilizing 0.1 to 0.5 weight percent of an ethoxylated fatty acid amide, such as polyoxyethylene (2) oleamide.
It would be highly advantageous if other additives obtained from readily available and economical materials were available and if coal-water slurries having high solids contents and which are stable for extended periods of time could be obtained by the use of these additives.